PL182297B1 - Surface-treated artificial reinforcements for wooden structural elements - Google Patents

Abstract

A synthetic reinforcement (22) having a plurality of micro-recesses (58) on at least one surface (54) to facilitate adhesion to at least one wood structural member (10) and a process for making the same is disclosed. The synthetic reinforcement (22) is comprised of a plurality of continuous fibers (46) that are maintained in position by a resin encasement (50). The surface (54) of the synthetic reinforcement (22) is characterized by micro-recesses (58) that are located in a generally random pattern, which increases the surface area of the resin encasement. The micro-recesses are formed by way of the addition of an agent such as a solid particulate, liquid or a multiplicity of gas-filled spheroids into the raw resin and then abrading away the agent in the surface of the cured resin. The synthetic reinforcement (22) is connectable to a wood laminae (10) or to another synthetic reinforcement with commercial grade adhesives such as the resorcinol resins, which are suitable for adhering the wood laminae together.

Description

The subject of the invention is a synthetic reinforcement for a wooden construction element, a method of producing technical, reinforced wooden construction elements and a wooden construction element.

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The invention relates to reinforcements for wooden construction elements such as beams, columns, panels and supports. In particular, the invention relates to synthetic reinforcements, each with a surface adapted to improved adhesion to wooden components. An object of the invention is to protect a given element from tensile or compressive stresses, or both, due to a high load on the element.

Beams, cantilevers, girders and columns are typical structural elements that carry loads or weight of structures, including buildings and bridges. Structural elements can be manufactured from a variety of materials such as steel, concrete and wood, depending on the structure design, environmental conditions and costs.

Today, timber construction members are typically made of a plurality of bonded wooden sections, such as glued laminates, laminated veneered lumber, parallel-joined timber, and I-beams. These types of technical wooden construction elements replace ordinary sawn timber, because the former have better construction parameters resulting from greater inspection possibilities and production control. Wood is a very desirable material for construction elements due to its favorable technical parameters, such as strength-to-weight ratio, appearance, reaction to cyclic loads and resistance to fire.

Laminated beams can be used as structural elements that span over open spaces, carrying loads of many tons. Typically, loading a laminated beam or beams with a uniform load distributed between the support points causes the bottom layer to be mainly in tension while the top layer is mainly in compression.

Synthetic wooden beam reinforcements can be constructed to withstand high tensile or high compressive stresses. The load-bearing capacity of laminated beams can be significantly increased by adding synthetic reinforcements in the areas of greatest stress, namely near the bottom and top layers. The condition for obtaining an appropriate reinforcing effect is usually the addition of various synthetic reinforcements in the areas of high tensile and compressive stresses.

There is a need for synthetic reinforcements that can be combined effectively and economically with the wood layers. Until now, only expensive epoxy resins have been used to connect plastic panels to wooden beams as well as to each other and other wooden construction elements. On the other hand, the layers of wood in wooden construction elements were usually joined with the use of cheap adhesives such as resorcinol, phenol-resorcinol, cross-linked melamine and polyvinyl acetate (PVA). Consequently, a separate gluing step and a separate gluing device are usually required to bond the synthetic reinforcements to the wood layers to obtain a reinforced glued laminated timber beam. Accordingly, it is an object of the invention to improve the adhesion of synthetic reinforcements to each other and to wooden construction elements, including laminated wooden elements.

The synthetic reinforcement for a wooden construction element comprising a plurality of filaments, substantially all of which are contained in a resin casing, is characterized according to the invention in that the resin casing has a major surface which includes a plurality of micro-cavities arranged substantially in a random pattern.

Preferably the fibers are aramid, glass, polyethylene or carbon fibers.

Preferably, the resin shell includes a plurality of micro-cavities arranged randomly therein.

Preferably, the resin casing has a large number of gas-filled bubbles randomly distributed therein.

It preferably further comprises a plurality of fibers each having an end extending from a surface of the resin sheath.

Preferably the resin shell is epoxy, polyester, vinyl ester or phenolic resin.

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Preferably, the resin shell is polyimides, PSP, PET, or polyamide 66.

Preferably, its thickness is less than 2 millimeters.

A synthetic reinforcement for bonding to a wooden construction element to increase its load-bearing capacity, comprising a plurality of filaments and a resin casing in which substantially all of the filaments are contained, is according to the invention characterized in that its thickness is less than 2 millimeters.

Preferably, it has a main surface covered with micro-cavities.

Preferably the fibers are aramid, glass, polyethylene or carbon fibers.

Preferably, the resin shell has a plurality of gas-filled bubbles randomly distributed therein.

It preferably further comprises a plurality of fibers with ends extending from the surface of the resin sheath.

Preferably the resin shell is epoxy, polyester, vinyl ester or phenolic resin.

Preferably, the resin shell is polyimides, PSP, PET, or polyamide 66.

The method of producing technical reinforced timber members with increased load-bearing capacity is distinguished according to the invention in that a synthetic reinforcement comprising a plurality of continuous filaments and a resin casing is provided which comprises substantially all filaments and has a surface in which there are many micro-cavities and interconnected by means of with an adhesive, at least one synthetic reinforcement with at least one of a plurality of bonded wood layers, the adhesive contacting at least a portion of the plurality of micro-cavities.

Preferably, the curable resin is cured and then the synthetic reinforcement is ground after curing such that some of the fibers protrude from the resin sheath.

Preferably, bubbles are formed in the resin.

Preferably, the agent is spread over the resin bath by wetting the plurality of filaments, the agent being a particulate solid.

Preferably, the agent is spread throughout the resin bath by wetting the plurality of filaments, and the agent having a large number of gas-filled, bubble-forming spheres is used.

Preferably the fibers are aramid, glass, polyethylene or carbon fibers.

Preferably the resin shell is epoxy, polyester, vinyl ester or phenolic resin.

Preferably, the resin shell is polyimides, PSP, PET, or polyamide 66.

The wood construction element is characterized according to the invention in that the first layer of wood is adhesively bonded to a set of adhesive-bonded synthetic reinforcements, each of which comprises a plurality of substantially continuous fibers along the length of the synthetic reinforcement, and a resin sheath substantially all of it. these fibers.

Preferably, the second wood layer is adhesively bonded to a set of adhesive-bonded synthetic reinforcements on the opposite side to the first wood layer.

Preferably, one additional synthetic reinforcement is bonded to the synthetic reinforcement by means of an adhesive.

Preferably it additionally comprises a parallel lumber unit.

Preferably, it additionally comprises veneered lumber.

Preferably, it additionally comprises an I-beam.

Preferably, each synthetic reinforcement has a main surface covered with micro-cavities.

Preferably the fibers are aramid, glass, polyethylene or carbon fibers.

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Preferably, the resin shell has a plurality of gas-filled bubbles randomly dispersed therein.

It preferably further comprises a plurality of fibers each having an end extending from a surface of the resin sheath.

Preferably the resin shell is epoxy, polyester, vinyl ester or phenolic resin.

Preferably, the resin shell is polyimides, PSP, PET, or polyamide 66.

It is an advantage of the invention to provide synthetic reinforcements with a treated surface, which facilitates the use of commercial adhesives such as resorcinol to bond the improved synthetic reinforcement to wood members.

Another advantage of the invention is to obtain an improved synthetic reinforcement comprising a resin shell with the main surface being micro-dimpled and increasing the surface area of the resin shell and increasing the adhesion to the substrate, thereby increasing the shear resistance of the adhesive.

The problems of the known reinforcement boards are solved in the present invention by using reinforcements with a surface adapted to greater adhesion to the wood laminates and to each other. The present invention provides a synthetic reinforcement that can include a plurality of continuous filaments in a resin sheath. On the surface of the resin shell there are numerous, substantially randomly distributed micro-cavities which increase the surface area of the resin shell. The depth and width of these micro-cavities are between one and two micrometers. The largest of them may be one or two millimeters wide and deep.

There can also be small spaces in the micro-cavities where adhesives can accumulate and polymerize, increasing the shear strength of the adhesives. The increased surface area of the synthetic reinforcement according to the invention can be bonded to the wood laminate or to each other by means of cheap commercial adhesives, including resorcinol resins such as those commonly used for bonding wood laminates to each other.

The present invention also relates to a process for producing reinforced wooden construction elements with increased load-bearing capacity. First, a synthetic reinforcement is prepared with the surface covered with micro-cavities. For this purpose, an auxiliary agent such as a volatile liquid or a particulate solid is spread in the resin to be set. Alternatively, a large number of gas-filled beads, each up to 2 mm in diameter, can be distributed throughout the resin. The spheres can be made of plastic, glass or any other material that can be satisfactorily shaped to the desired shape.

A plurality of continuous filaments are then wetted with this blend and the resin is fixed. If the medium is a liquid, it generally becomes gaseous during fixation, leaving a large number of bubbles in the surface of the plate. The surfaces of the reinforcement element to be bonded to the layer are then ground to remove bubbles from their top layer if the medium used is a liquid or multiple gas-filled spheres, or to remove embedded solids therefrom if the medium used is a particulate matter constant. After the final stage, there is a random pattern of micro-cavities.

The surface-modified reinforcement board may be bonded to one or more wood members or other reinforcement boards or layers such that the adhesive contacts at least a portion of the plurality of micro-cavities.

Furthermore, the present invention includes optimizing the length of time between the application of the adhesive layer on the first surface and the contact of the second surface with the adhesive layer. The length of this time is referred to as "open time". The optimization of the open time according to the invention increases the mechanical strength and the cohesion of the wood and the reinforcement layer.

The subject of the invention is illustrated in the drawing in which fig. 1 shows a laminated timber beam with synthetic reinforcements according to the drawing

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In one embodiment, the liquid agent 62 may be methyl ethyl ketone or toluene which is added preferably to the stabilized resin in an agent to resin weight ratio of 2 to 15 percent. In one specific preferred embodiment, the agent is toluene added to the stabilized resin at a concentration of about 5% by weight.

An additional method of producing the micro-cavities 58, shown in Figure 6, is to add to the resin bath (not shown) a large number of gas-filled bubbles 64 having diameters in the range of about 10 to 2000 micrometers. Gas-filled bubbles 64 are widely available and are sometimes used for packaging purposes. They can be made from a wide variety of materials, including thermosetting and thermoplastic resins. After the fibers 46 are immersed in the resin bath and the resin 50 is set, the resin surface is slightly rubbed, sufficient to remove the exposed portions of the bubbles 64 and leave micro-cavities 58 in the surface of the fixed resin. synthetic reinforcement 22 during the setting process, so that fewer bubbles 64 remain inside the reinforcement 22.

Synthetic reinforcements 22 thus produced may have a thickness of less than 2 mm. As already mentioned, if one of these reinforcements 22 is not strong enough, several are glued together to form a set 22a or 22b which together has adequate mechanical strength.

Another preferred method of producing micro-cavities 58 in the surface 54 of the resin casing 50 is by grinding the surface 54. This can be done by scrubbing it with a grit in a perpendicular or longitudinal direction to the longitudinal direction of the reinforcement 22. Sanding the surface 54 of the resin casing 50 removes a small portion of the set resin. Sanding the surface 54 of the resin casing 50 can remove the particulate center 60 or expose the pores formed by the liquid or solid center 62 which, when exposed, transform into micro-cavities 58. The adhesive can then penetrate into the micro-cavities 58 to form a strong and resilient bond between the reinforcement 22 and the beam layer 18. 10, 14 or with some other structural element. The reinforcements 22 according to the invention can be produced by a continuous pressing technique.

The experiments carried out led to the important discovery that open times can be optimized. Open time is the time during which the adhesive used for the interlayer between the reinforcement and the wood layer is allowed to air dry and penetrate the surface before it comes into contact with the other bonding surface. After the adhesive is applied to the surface of the synthetic reinforcement board, where it is to form an intermediate layer between the reinforcement and the wood layer, the surface is usually left covered with the adhesive for a short time, open time, undisturbed. Typical testing times were 5 to 10 minutes; for the production process, longer open times are usually used.

After the open time has elapsed, the glued surface is brought into contact with the corresponding surface of the wood layer, which can also be covered with glue and also left open for a while. According to a preferred embodiment of the invention, the open time for reinforcements is preferably greater than 10 minutes, but less than 80 minutes and most preferably 30 minutes. In some applications, such as the production of larger beams, an open time greater than 80 minutes may be better. After the open time has elapsed and the surface covered with the adhesive has come into contact with the desired surface, the reinforcement and the wood layer are usually pressed together, ensuring an optimal bonding at a pressure above 861.85 kPa, resulting in an adhesive layer less than 0.10 mm thick.

Typical holding time is 8 to 10 hours, provided there is no post-clamping period after the clamping before the reinforced structural member is stressed. It has been found that the post-pinch setting time increases the mechanical strength of the bond and enables more accurate fixation.

Fig. 2, a laminated wooden beam with synthetic reinforcements according to the invention located on the outer surfaces, in a side view, Fig. 3, an improved reinforced slab according to the invention, in a perspective view, 4A and 4B - an improved reinforced plate according to the invention, disassembled into its components at a high magnification, with solid particles on the resin shells shown in Fig. 4A and with micro-cavities after removal of the solid particles visible in Figs. 4B, Figs. 5A-5C. - an alternative, improved reinforced sheet according to the invention, at high magnification, with solid particles or liquid droplets on the reinforcement fibers visible in Fig. 5A, with a resin sheath as shown in Fig. 5B, and with micro-depressions formed in Fig. 5C. after removal of liquid droplets or solid particles, and Figs. and at high magnification, with the plurality of gas filled beads visible in Fig. 6A in a partially completed reinforcement. Fig. 6B shows the reinforcement after setting the resin and after sandblasting the surface, which leaves micro-pits.

Figures 1 and 2 show glue-laminated timber construction members 10 and 14 composed of a plurality of bonded wood layers 18, preferably in the form of long boards. In the arrangement shown here, timber beams 10 and 14 are glue-laminated lumber according to American Institute of Timber Construction (AITC) manufacturing standards 117-93 of Engelewood, Colorado. The illustrated arrangement is the preferred arrangement for timber beams 10 and 14, but the following description also applies to other timber construction members including veneered lumber, laminated timber, timber I-beams, and reinforced timber members.

As can be seen in Fig. 1, the first set of synthetic reinforcements 22a is between the bottom layer 26 and the adjacent layer 30. The second set of synthetic reinforcements 22b is between the top layer 34 and the adjacent layer 38. Both sets of reinforcements 22a and 22b, hereinafter referred to as collectively, as reinforcements 22, they extend approximately three-fifths of the length of beam 10. The reinforcements may extend over most of beam 10 or the entire length of beam 10.

As with the examples of straight beams, the timber members 10 and 14 may be supported by a pair of supports 39 and bear the load 40. In this situation, the sets of synthetic reinforcements 22a and 22b are placed in areas of high tensile and high compressive stresses, respectively. It goes without saying that the sets of reinforcements 22a and 22b may be located in areas of high compressive and tensile stresses, respectively, as long as the beam structural members 10 and 14 are cantilevered.

Spacers 42 extend from the end of each of the reinforcement assemblies 22a and 22b to the end of the wooden beam 10 and are preferably made of wood. A reinforcement kit covering two-fifths to three-fifths of the central part of the beam provides virtually all the major benefits of a full-length reinforcement kit, but is less expensive per beam. Fig. 2 shows one alternative embodiment of the present invention in which sets of reinforcements 22a and 22b are provided on the outer surface of wooden beam 14 and have no spacers.

Figure 2 is an enlarged single reinforcement 22 which is shown to include a plurality of synthetic fibers 46 extending substantially parallel to each other and approximately in the direction of the longitudinal dimension of the reinforcement 22 as described in detail hereinafter. The arrangement and distribution of the synthetic fibers 46 hold the resin sheaths 50 surrounding the fibers and filling the spaces therebetween. In one preferred embodiment, resins are used that are set, low-cost, commercially available adhesives, such as, for example, resorcinol resins, phenol resorcinol resins, cross-linked melamine, and polyvinyl acetate (PVA) suitable for bonding the wood laminate 18 (and reinforced wood composites) with each other.

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Preferably, the reinforcement fibers are aramid, glass or carbon fibers. Aramid fibers are available from E. I. duPont de Nemours & Co., Delaware under the tradename Kevlar ™ and from the Enka BV subsidiary of Akzo N.V. (Arnhem, The Netherlands) under the trade name Twaron ™. The preferred grade of aramid fibers is Kevlar 49 ™. The fibers 46 may also be or include high modulus polyethylene fibers, such as Spectra ™ high molecular weight fibers sold by Allied Fibers of Allied Signal, St. Petersburg, Va. Another fiber that may be used is S-2 glass fibers from Owens-Coming Fiberglass, Corning, New York. The preferred materials for use in areas with high tensile and high compressive stresses are aramid fibers and carbon fibers, respectively. Glass fibers are a cheaper alternative to both.

Experiments with non-epoxy resin casings have shown poor interlayer shear strength in reinforcement 22. Preferably, the set resin used to produce reinforcement 22 is an epoxy resin. However, in alternative embodiments, other resins such as polyester vinyl ester and phenolic resins may be used. In alternative embodiments of this invention, thermoplastic resins such as polyethylene terephthalate (PET), PSP, or polyamide 66 may be used.

In many situations, the single reinforcement 22 provides sufficient mechanical strength and modulus of elasticity to ensure that the laminate meets the specified requirements, but it is also possible to bond the set of synthetic reinforcements 22a or 22b together into a laminate as clearly shown in Figures 1 and 2. Described herein the synthetic reinforcements 22 can be bonded together without the use of epoxy resins.

According to the invention, on the opposing major surfaces 54 and 56 of the reinforcement 22, a plurality of micro-cavities 58 are provided in the resin skin 50 in the resin jacket. The micro-cavities 58 increase the surface area of the reinforcement 22, facilitate the adhesion of the reinforcement 22 to the adjacent wood layer 18 and to each other in multilayer laminates, and increase mechanical strength of the adhesive joint.

To date, the reinforcements 22 have been produced by wetting the fibers in a resin bath and then fixing the resin sheath 50. In a preferred method of producing reinforcement elements with micro-cavities 58 in the surface, an excipient is mixed with a resin bath and then the fibers are moistened with a mixture of resin and an excipient to form a sheath 50.

Figures 4A and 4B show that the agent may be in the form of particulate solid particles 60 such as chalk dust or clay. This agent is removed by lightly rubbing the major surfaces 54 and 56 after fixing the resin sheath 50, resulting in micro-depressions 58. Typically, the width and depth of the micro-depressions are in the order of a few micrometers. The maximum width and depth of the micro-cavities may be one to two millimeters.

One disadvantage of this method is that some of the particulate solids remain mixed with the resin 50, which degrades the mechanical properties of the set resin 50. However, generally the better adhesion of the reinforcement 22 more than compensates for any deterioration in the mechanical properties.

In an alternative production method described with reference to Figs. 5A-5C, the agent may be a spray of liquid material 62 added to a resin bath (not shown) before wetting the reinforcement fibers 46. For example, the liquid agent 62 may be selected such that its boiling point was less than that selected for setting the resin casing 50. After wetting the fibers 46 in the bath, during the next step of fixing the resin, the material emits, forms, or bubbles into non-resin-reactive gas. These bubbles tend to migrate towards the surface of the synthetic reinforcement where they either break to form micro-cavities 58 or remain bubbles during setting. After the resin is cured, the surface of the material is lightly rubbed to remove the exposed surfaces of the remaining bubbles, thereby creating additional micro-cavities 58.

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50

FIG. 5α 46

FIG. 5b 46

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FIG. 3

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FIG. 2

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Publishing Department of the Polish Patent Office. Circulation of 60 copies. Price PLN 4.00.

Claims (35)

Patent claims 1.A synthetic reinforcement for a wooden construction element comprising a plurality of filaments substantially all contained in a resin casing characterized in that the resin casing (50) has a major surface (54, 56) which has a plurality of micro-cavities (58) arranged basically in a random pattern. 2. The reinforcement according to claim The fiber of claim 1, wherein the fibers (46) are aramid, glass, polyethylene or carbon fibers. 3. The reinforcement according to claim The resin casing (50) as claimed in claim 1, wherein the resin shell (50) comprises a plurality of micro-cavities (58) distributed therein in a random pattern. 4. The reinforcement according to claim The resin casing (50) comprises a plurality of gas-filled bubbles (64) randomly distributed therein. 5. The reinforcement according to claim 1 The apparatus of claim 1, further comprising a plurality of fibers (46) each having an end extending from a surface of the resin casing (50). 6. The reinforcement according to claim The resin of claim 1, wherein the resin casing (50) is epoxy, polyester, vinyl ester or phenolic resin. 7. The reinforcement according to claim The resin casing (50) as claimed in claim 1, wherein the resin casing (50) is polyamides, PSP, PET or polyamide 66. 8. The reinforcement according to claim The method of claim 1, wherein its thickness is less than 2 millimeters. 9. Synthetic reinforcement for bonding to a wooden construction element increasing its load-bearing capacity, comprising a plurality of filaments and a resin casing in which substantially all the filaments are contained, characterized in that its thickness is less than 2 millimeters. 10. The reinforcement according to claim 1 The device of claim 9, characterized in that it has a main surface (54, 56) covered with micro-cavities (58). 11. The reinforcement according to claim 1 9. The method of claim 9, characterized in that the fibers (46) are aramid, glass, polyethylene or carbon fibers. 12. The reinforcement according to claim 1 The resin casing (50) comprises a plurality of gas-filled bubbles (64) randomly distributed therein. 13. The reinforcement according to claim 1 The apparatus of claim 9, further comprising a plurality of fibers (46) with ends protruding from the surface of the resin casing. 14. The reinforcement according to claim 1 The method of claim 9, wherein the resin casing (50) is epoxy, polyester, vinyl ester or phenolic resin. 15. The reinforcement according to claim 15 9. The resin casing (50) as claimed in claim 9, wherein the resin casing (50) is polyimide, PSP, PET or polyamide 66. 16. A method of producing technical reinforced timber members with increased load capacity, characterized by providing a synthetic reinforcement (22) comprising a plurality of continuous filaments (46) and a resin casing (50) comprising substantially all of the filaments (46) and having a surface (54, 56) having a plurality of micro-cavities (58) and adhesively bonding at least one synthetic reinforcement (22) to at least one of the plurality of bonded wood layers (18), the adhesive contacting at least one part of a plurality of micro-cavities (58). 182 297 17. The method according to p. The method of claim 16, wherein the curable resin is cured and then the synthetic reinforcement (22) is ground after curing such that a portion of the fibers (46) protrude from the resin casing (50). 18. The method according to p. The process of claim 16, wherein bubbles (64) are formed in the resin. 19. The method according to p. The process of claim 16, wherein the agent is dispersed in a resin bath by wetting the plurality of filaments (46), a particulate solid (60) being used as the agent. 20. The method according to p. The method of claim 16, wherein the agent is dispersed in the resin bath by wetting the plurality of filaments (46), wherein the agent comprises a plurality of gas-filled bubble-forming spheres (64). 21. The method according to p. 16. The process of claim 16, wherein the fibers (46) are aramid, glass, polyethylene or carbon fibers. 22. The method according to p. The method of claim 16, wherein the resin casing (50) is epoxy, polyester, vinyl ester or phenolic resin. 23. The method according to p. 16. The resin casing (50) is polyimide, PSP, PET or polyamide 66. A wooden construction element, characterized in that the first layer (18) of wood is adhesively bonded to a set of adhesive-bonded synthetic reinforcements (22), each of which comprises a plurality of substantially continuous fibers (46) running along the length of the synthetic reinforcement. (22) and a resin sheath (50) encompassing substantially all of these fibers. Construction element according to claim 25. A method as claimed in claim 24, characterized in that the second wood layer (18) is adhesively bonded to a set of glue-joined synthetic reinforcements (22) on the opposite side to the first wood layer (18). 26. Construction element as claimed in claim 26. The method of claim 24, characterized in that at least one additional synthetic reinforcement is bonded to the synthetic reinforcement (22) by means of an adhesive. 27. Construction element as claimed in claim 27. 24, characterized in that it further comprises a parallel unit of lumber. 28. Construction element as claimed in claim 28 24, characterized in that it further comprises veneered lumber. 29. Construction element as claimed in claim 29. 24, characterized in that it further comprises an I-beam. Construction element according to claim 1 The method of claim 24, characterized in that each synthetic reinforcement (22) has a major surface (54, 56) covered with micro-cavities (58). 31. Construction element as claimed in claim 31. The method of claim 24, wherein the fibers (46) are aramid, glass, polyethylene or carbon fibers. 32. Construction element according to claim 32. The method of claim 24, wherein the resin shell (50) has a plurality of gas-filled bubbles (64) randomly dispersed therein. 33. Construction element according to claim 33. The method of claim 24, further comprising a plurality of fibers (46) each having an end extending from a surface of the resin casing (50). 34. Construction element according to claim 34. The method of claim 24, wherein the resin casing (50) is epoxy, polyester, vinyl ester or phenolic resin. Construction element according to claim 35. The process of claim 24, wherein the resin casing (50) is polyimide, PSP, PET, or polyamide 66. ♦ $ $